Spring loaded chain tensioner that keeps chain from popping and snapping. Can be used for all chain drive applications including center mount and rear mount chain drive for gas motorized bicycle plan kits. Must be mounted so that it pulls on the chain so the spring action will work effectively keeping the chain tight, but not too tight. Spring loaded installation positions on bicycle so the spring pulls on the chain. Can also be placed on the lower chain. The amount of tension can be adjusted to your liking. Remove vibration and snapping and popping of the chain during operation. It also keeps the nylon tension wheel from wearing out. This makes for a smoother, more enjoyable ride without vibration and increases the lifespan of not only the chain, but motorized bicycle parts. Makes the whole bike, motor and everything else last longer, because you aren"'"t shaking it excessively. It really makes the difference between enjoying your ride and being glad that it was over.